This application relates to the operation of re-programmable memory systems such as semiconductor flash memory systems, and, more specifically, to systems and methods for managing power in such memory systems when a power supply voltage drops.
Solid-state memory capable of nonvolatile storage of charge, particularly in the form of EEPROM and flash EEPROM packaged as a small form factor card, has recently become the storage of choice in a variety of mobile and handheld devices, notably information appliances and consumer electronics products. Unlike RAM (random access memory) that is also solid-state memory, flash memory is non-volatile, and retains its stored data even after power is turned off. Also, unlike ROM (read only memory), flash memory is rewritable similar to a disk storage device. In spite of the higher cost, flash memory is increasingly being used in mass storage applications. Conventional mass storage, based on rotating magnetic medium such as hard drives and floppy disks, is unsuitable for the mobile and handheld environment. This is because disk drives tend to be bulky, are prone to mechanical failure and have high latency and high power requirements. These undesirable attributes make disk-based storage impractical in most mobile and portable applications. On the other hand, flash memory, both embedded and in the form of a removable card is ideally suited in the mobile and handheld environment because of its small size, low power consumption, high speed and high reliability features.
Flash EEPROM is similar to EEPROM (electrically erasable and programmable read-only memory) in that it is a non-volatile memory that can be erased and have new data written or “programmed” into their memory cells. Both utilize a floating (unconnected) conductive gate, in a field effect transistor structure, positioned over a channel region in a semiconductor substrate, between source and drain regions. A control gate is then provided over the floating gate. The threshold voltage characteristic of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, for a given level of charge on the floating gate, there is a corresponding voltage (threshold) that must be applied to the control gate before the transistor is turned “on” to permit conduction between its source and drain regions. In particular, flash memory such as Flash EEPROM allows entire blocks of memory cells to be erased at the same time.
In addition to flash memory, other forms of nonvolatile memory may be used in nonvolatile memory systems. For example Ferroelectric RAM (FeRAM, or FRAM) uses a ferroelectric layer to record data bits by applying an electric field that orients the atoms in a particular area with an orientation that indicates whether a “1” or a “0” is stored. Magnetoresistive RAM (MRAM) uses magnetic storage elements to store data bits. Phase-Change memory (PCME, or PRAM) such as Ovonic Unified Memory (OUM) uses phase changes in certain materials to record data hits. Various other nonvolatile memories are also in use or proposed for use in nonvolatile memory systems.
A common memory system comprises one or more memory chips that are mounted on a printed circuit board, along with a memory controller chip, and enclosed in a housing to form a memory card, Universal Serial Bus (USB) flash drive, or Solid State Drive (SSD) that has a physical interface that allows it to be connected with a range of host devices. Typically, the memory system receives its power from a host through such an interface so that it is dependent on the host for its power. The voltage at which electrical power is supplied by the host may be set by an interface standard (e.g. USB provides a 5.0 volt power supply). There is generally some range of acceptable supply voltage that the host is expected to provide. If the voltage drops below that range, the memory system may reset causing all ongoing memory operations to stop, potentially causing loss of data that is not yet saved, and requiring time and power to recover if the power supply voltage returns to its specified range.
In mobile devices, power is typically provided by a battery. Current may be drawn from the battery by different components at different times so that the power supply voltage may drop as significant current is drawn. Because battery life is an important feature in mobile devices, manufacturers attempt to extend the battery life by continuing to run the mobile device as the battery nears exhaustion. However, the voltage supplied by the battery may drop from its nominal value in this near-exhaustion condition. Multiple power demands make it difficult for a mobile host to provide a power supply at a consistent voltage to a memory system especially as the battery nears exhaustion. Fluctuations in the power supply voltage may cause problems even before the power supply voltage reaches a point where a reset is triggered. Therefore, there is a need for a power management system for dealing with fluctuations in power supply voltage, particularly in mobile devices.